Children's Exposure to Flame Retardants: Effects on Thyroid Hormone Regulation. The flame retardant chemicals polybrominated diphenyl ethers (PBDEs) are persistent and ubiquitous pollutants, particularly in indoor environments. Human biomonitoring studies have shown that levels of PBDEs are more than an order of magnitude higher in the United States population relative to any other region measured, including other industrialized countries in Europe. Furthermore, children between the ages of 0-4 are predicted to have the highest exposure and body burdens of PBDEs due to placental transfer, breast milk ingestion and inadvertent dust ingestion. Children's exposure is of concern because laboratory studies using animal models have found that PBDEs exert negative effects on thyroid hormone homeostasis, which is more pronounced in newborn animals relative to adults. The central hypothesis of this proposed study is that children in the United States are receiving greater exposure to PBDEs from inadvertent dust ingestion, leading to adverse effects on thyroid hormone regulation. The objectives of this study are threefold: 1) to measure children's (ages 1-4) exposure to PBDEs from indoor dust, determine ingestion rates from hand to mouth contact, and measure serum levels of PBDEs in these children;2) to determine the products of hepatic metabolism of PBDEs;and 3) to determine the effects of PBDEs and their metabolites on intra- and extra-cellular thyroid hormone regulation to elucidate the mechanisms of action for thyroid toxicity. Using in vitro assays the following will be investigated: the effects of PBDEs and their metabolites on intracellular thyroid hormone levels, deiodinase (DI) enzyme activity, mRNA expression of thyroid regulating genes, and specific binding to thyroid binding globulin. The principal investigator also plans to develop and validate a new method to measure thyroid hormone levels and DI activity using liquid chromatography tandem mass spectrometry (LC/MS-MS), which will use stable isotopes instead of radio-labeled isotopes used in more traditional methods. This method could then be used as a screening tool for assessing effects of chemicals on intra-cellular thyroid regulation mediated by DI enzymes. Data gathered from this project will increase the understanding of PBDE effects on thyroid hormone regulation and aid in understanding children's developmental health risks from exposure to flame retardant chemicals in indoor environments.